It is well established that a correlation exists between elevated levels of serum cholesterol and the development of cardiovascular disease (CVD). Four major circulating lipoproteins have been identified in serum including chylomicrons (CM), very low density (VLDL), low density (LDL) and high density (HDL) lipoproteins. LDL and HDL are the major cholesterol carriers. VLDL and LDL have been shown to be responsible for cholesterol transport from the liver (where it is synthesized or obtained from dietary sources) into extrahepatic tissues in the body, including arterial walls. In contrast, HDL is directly involved in the removal of cholesterol from peripheral tissues, carrying it back to the liver or to other lipoproteins by a mechanism known as reverse cholesterol transport (RCT) (reviewed in Oram, J. F., Arterioscler. Thromb. Vasc. Biol. 23:720-727, 2003; Oram, J. F., et al., Phys. Rev. 85:1343-1372, 2005). The protective role of HDL has been shown in a number of studies in which high levels of HDL seem to confer cardiovascular protection (see, e.g., Miller, et al., Lancet 1(8019):965-968, 1977; Whayne et al., Atherosclerosis 39:411-419, 1981). It is hypothesized that high levels of plasma HDL are not only protective against coronary artery disease, but may actually induce regression of atherosclerotic plaques (see, e.g., Badimon, et al., Circulation 86(Supp. III):86-94, 1992).
Recent interest in the protective mechanism of HDL has focused on apolipoprotein Al (apoA-I), the major component of HDL. High plasma levels of apoA-I are associated with reduced risk of CVD and less frequent presence of coronary lesions. (See, e.g., Maciejko et al., N. Engl. J. Med. 309:385-389, 1983; Sedlis et al., Circulation 73:978-984, 1986.) Genetic deficiencies in apoA-I are associated with abnormalities in lipoprotein metabolism that result in low plasma HDL levels, intracellular cholesterol accumulation and premature atherosclerosis. Overexpression of apoA-I in transgenic mice and rabbits increased HDL levels and reduced CVD (von Eckardstein, A., et al., Atherosclerosis 137(Supp. S):S7-11, 1998).
Lipid-depleted apoA-I removes excess cholesterol and phospholipids from cells such as macrophages through its interaction with a cell membrane protein called ATP-binding cassette transporter A1 (ABCA1) (Oram, J. F., et al., Physio. Rev. 85:1343-1372, 2005). This process has broad specificity for multiple exchangeable HDL apolipoproteins (Remaley, A. T., et al., Biochem. Biophys. Res. Commun. 280:818-823, 2001; Segrest, J. P., J. Lipid Res. 33:141-166 (1992)). This process is believed to be the rate-limiting step in the generation of mature HDL particles. Studies of human patients and animal models have shown that ABCA1 is cardioprotective. For example, loss-of-function mutations in human ABCA1 cause a severe HDL deficiency syndrome characterized by deposition of cholesterol in tissue macrophages and prevalent CVD. Ablating the ABCA1 gene in mouse macrophages increases atherosclerosis, and increasing ABCA1 expression in mice decreases atherosclerosis. The interaction of apoA-I with ABCA1 or ABCA1-expressing cells elicits several responses involved in exporting cellular cholesterol: removing cholesterol and phospholipids that are transported to the cell surface by ABCA1, stabilizing ABCA1 protein so that it has sustained activity, and stimulating cellular signaling pathways that control ABCA1 activity. In one of these signaling pathways, apoA-I rapidly activates a tyrosine kinase called Janis kinase 2 (JAK2), which promotes the apoA-I binding to ABCA1 necessary for cholesterol removal (Tang, C., et al., J. Biol. Chem. 279:7622-7628, 2004). Gene transcription of ABCA1 is highly induced by cellular cholesterol.
Oxidative damage is implicated in the pathogenesis of atherosclerosis, a chronic inflammatory disease. Early atherosclerotic lesions are rich in phagocytic cells, which are predominantly macrophages. Macrophages contribute to the inflammatory process by producing reactive oxygen species such as superoxide and H2O2 (Klebanoff, S. J., Ann. Intern. Med. 93:480-489, 1980). These intermediates can be converted to more the powerful oxidants HOCl and peroxynitrite through a pathway involving myeloperoxidase (MPO), a heme protein released by macrophages via the following reaction: H2O2+Cl−+H+→HOCl+H2O (Harrison, J. E., et al., J. Biol. Chem. 251:1371-1374, 1976). The physiological importance of this reaction is underlined by the presence of enzymatically active MPO in human atherosclerotic lesions (Daugherty, A., et al., J. Clin. Invest. 94:437-444, 1994). Moreover, HOCl-modified lipoproteins have been detected in advanced human atherosclerotic lesions (Hazell, L. J., et al., J. Clin. Invest. 97:1535-1544, 1996).
Loss of the ability of apoA-I to remove cholesterol from cells by the ABCA1 pathway is strongly associated with modification of specific amino acid residues in apoA-I (Bergt, C., et al., Proc. Nat'l Acad. Sci. 101:13032-13037, 2004; Shao, B., et al., J. Biol. Chem. 280:5983-5993, 2005). ApoA-I in human atherosclerotic lesions is modified by acrolein (Shao, B., et al., J. Biol. Chem. 280:5983-5993, 2005), a reactive carbonyl generated metabolically and by lipid peroxidation. ApoA-I isolated from atherosclerotic lesions is modified by reactive chlorine and nitrogen species as well as by reactive carbonyls (Pennathur, S., et al., J. Biol. Chem. 279:42977-42983, 2004; Shao, B., et al., J. Biol. Chem. 279:7856-7866, 2004). ApoA-I contains four tryptophan residues, and this aromatic amino acid is very sensitive to oxidative damage (Fu, X., et al., Biochemistry 45(12):3961-71, 2006; Fu, X., et al., J. Biol. Chem. 279(8):6209-12, 2004; Fu, X., et al., J. Biol. Chem. 278(31):28403-9, 2003). Thus, oxidation of specific amino acid residues in apoA-I is one mechanism for loss of its biological activities. The underlying factors that may initiate or promote these modifications of apoA-I include inflammation and diabetes, a disorder characterized by elevated levels of reactive carbonyls and a greatly increased risk of atherosclerotic vascular disease (Baynes, J. W., et al., Free Radic Biol Med 28(12):1708-16, 2000; Baynes J. W. et al., Diabetes 48(1): 1-9, 1999).
Animal studies indicate that synthetic amphipathic peptides based on the structural motifs of apoA-I exert potent anti-inflammatory, anti-dyslipidemic and anti-atherogenic effects (Navab, M., et al., Trends Cardiovasc. Med. 15(4):158-61, 2005; Navab, M., et al., Curr. Opin. Investing Drugs 4(9):1100-4, 2003). ApoA-I and other apolipoproteins inhibit neutrophil activation (Terkeltaub, R. A., et al., J. Clin. Invest. 87(1):20-6, 1991; Blackburn, W. D. Jr., et al., J. Lipid Res. 32(12):1911-8, 1991; and Martinon, F., et al., Nature 440(7081):237-41, 2006), indicating that apoA-I or peptides based on the sequence or structure of apoA-I may have therapeutic effects in inflammatory conditions mediated by activated leukocytes (neutrophils, monocytes, macrophages, eosinophils, mast cells and basophils). Leukocytes are of central importance in disorders such as arthritis and other rheumatological conditions as well as a wide range of other acute and chronic inflammatory conditions (Kaneider, N. C., et al., F.E.B.S. J. 273(19):4416-24, 2006; Nakamura, K., et al., World J. Gastroenterol. 12(29):4628-35, 2006; Serhan, C. N., et al., Nat. Immunol. 6(12):1191-7, 2005; Henson, P. M., Nat. Immunol. 6(12):1179-81, 2005; and Hoffman, M., et al., Atherosclerosis 172(1):1-6, 2004).
Recently, intense interest has developed in using HDL or apoA-I to treat or prevent cardiovascular disease. However, recent studies have shown that cardioprotective effects of HDL and apoA-I may be lost when HDL is oxidatively modified in vivo. The present inventors have determined that mutant apoA-I protein or synthetic peptides resistant to oxidation, reactive carbonyls, or other reactive intermediates promote cholesterol efflux, and therefore may be effective at preventing or treating inflammatory disorders such as arthritis, inflammatory bowel disease, and acute coronary syndrome.